Electrical fuses and circuit interrupters have been used to disconnect electrical power supplies when overcurrent and/or overvoltage situations occur. Even the most accurate fuses and circuit interrupters require a substantial response time (i.e. 10 seconds to 100 seconds at twice the current limit) between the time the malfunction is detected and the time the electrical supply is switched off. Sensitive electrical and electronic components currently require a much faster response time to be adequately protected. Other types of equipment must also be quickly shutdown in response to detection of physical parameters, such as pressure, temperature, speed etc., when those parameters exceed a predetermined level.
In addition, people with heart problems are not able to withstand a current flow of 6 to 300 mA during the response time of currently available current interruption devices. During the response time, the current flow may increase to some multiple of the threshold detection current. However, people with heart problems will be harmed if exposed to ground fault current for more than 20 ms. In such a circumstance, a car pacemaker may malfunction and cause ventricular fibrillation.
There have been many attempts in the prior art to produce a device which will prevent accidental electrocution. These devices work in a number of ways. Some operate by quickly detecting and reacting to a resultant overload. Others react to a short between a supply conductor and an external ground. Another type of device detects a short occurring in the load where a person may be connected between the shorted load and the ground. Finally, there have also been many attempts to provide overload and/or over-voltage protection for circuits.
There are two types of existing devices of particular interest in relation to the present invention. First, there is the Immersion Detection Circuit Interrupter ("IDCI"). Second, there is the Ground Fault Circuit Interrupter ("GFCI"). Both of these devices can be physically located in either the power outlet, in the plug, or in the apparatus itself.
A modification of the GFCI type device is described in applicant's co-pending U.S. patent application Ser. No. 07/621,342, the text of which is incorporated herein by reference.
The types of devices described above (i.e. IDCI, GFCI, and the modified GFCI) typically utilize mechanical circuit interrupters, switches, or other contacts in order to disconnect the power supply from the apparatus. These mechanical-type devices suffer from the disadvantage of a high failure rate due to sticking or jamming of the movable parts comprising the device. In addition, these mechanical type switches are expensive in terms of both material and assembly costs.
Electronic-type switches such as described in U.S. Pat. No. 3,997,818 have also been previously utilized. These types of switches are also high in cost. In addition, these switches suffer from the disadvantage of generating a large amount of waste heat in the switch components during normal operation. In appliances requiring a high load current flow, such as a hair dryer, the waste heat generated by the switch can be excessive. These electronic circuit interrupters also suffer from the disadvantage of being susceptible to transinter alloy formation. Transinter alloy formation occurs when two alloys comprising different parts of the switch diffuse into each other. When this phenomenon occurs, the switch will no longer be able to perform its function of shutting off power to the apparatus since the switch will be permanently closed.
Another type of circuit interruption device which has been previously utilized combines conductive material in combination with explosives. Such devices are described in U.S. Pat. No. 3,728,583 and PCT/EP/00226. A typical device of this sort includes a capacitor which discharges to a thin wire. Upon discharge of the capacitor, the thin wire evaporates instantaneously and causes an explosion which destroys a conductive connector. Destruction of the conductive connector causes the power to the electrical apparatus to be terminated. This type of device requires components which are both responsive to electrical signals and also destructible. This type of device is complicated to design since the remnants of the destroyed components need to be contained to prevent shorts or electrical connections after the safety switch has been triggered. This complicated design may render these types of switches even more costly to assemble than the mechanical switches described above.